Dry concentrator



Mardi 5, 1963 P. s. DREYFUS ETAL 3,089,056

DRY coNcENTRAToR Filed Nov. 29, 1960 2 Sheets-Sheet 1 Mardi 5, 1953 P. s. DREYFUS ErAL 3,080,056

DRY CONCENTRATOR Filed NOV. 29, 1960 @i7 59 @la 2 Sheets-Sheet 2 9 if @l if 5/ 562/ l5 74 3,086,055 DRY CONCENTRATGR Philip S. Dreyfus, Chicago, lli., and Lester G. Fernstroni, Tucson, Ariz.; The First National Bank of Chicago, executor of said Philip S. Dreyfus, deceased Filed Nov. 29, 1960, Ser. No. 72,462 4 Ciainxs. (Ci. 269-466) This invention relates to improvement in a dry concentrator and, more particularly, to aconcentrating table having fifties of a novel construction for separating valuable material from waste material While in a dry state.

It is an object of the present invention to provide a table for the separation of a mass of material, composed of parts having diiierent characteristics, such as the yield from mines and the like, in order to concentrate the valuable particles therein and to separate the same from the waste particles, without requiring the use of water.

It is another object of the present invention to provide a dry concentrator for the separation of mixed particles of sized and graded material according to the specic gravity of such particles.

It is a further object of the present invention to provide a dry concentrator for operation upon a continuously owing stream of mixed material, commonly referred to as heads, so as to eiiect a separation of the valuable and waste particles from each other.

It is yet a further object of the present invention to provide a dry concentrator for the recovery of valuable particles from mixed materials by the use oi an apparatus which minimizes the loss of valuable particles, thereby reducing the concentrating costs,

It is a still further object of the present invention to provide a dry concentrator comprising a separation table having means for uniform air oatation in combination with a plurality of spaced rifes of a novel construction, thereby producing a percentage recovery not possible with the type of dry concentrating table available prior to the present invention.

Other objects and features of novelty will be apparent from the following specication, when read in connection with the accompanying drawings, in which certain embodiments of the invention are illustrated by way of example.

In the drawings:

FIGURE l is a perspective view of one embodiment of a dry concentrating table embodying the principles of the present invention, and specifically illustrating the introduction of heads at the upper side and head end of the table, the discharge of gangue or tailings from the lower side of the table, and the capture of concentrate and middlings from the foot of the table;

FIGURE 2 is a reduced rscale perspective view from the left rear of the dry concentrator shown in FiGURE l, specically illustrating the means for introducing uniform air pressure to the deck of the table;

yElGURE 3 is a reduced scale perspective view of the foot of the dry concentrator illustrated in FIGURE l, specifically illustrating the means for regulating the angle of inclination of the table;

FIGURE 4 is an enlarged scale top plan view of the table deck of the dry concentrator shown in FIGURE l, specifically illustrating the alignment of the riles in this embodiment of the invention;

:idd Faterited Mar. 5, SSB

FIGURE 5 is a sectional view taken along the line 5 5 of FIGURE 4;

FIGURE 6 is a cross-sectional view along the line 6 6 of FIGURES 4 and 5;

FIGURE 7 is a greatly enlarged cross-sectional View of any one riie along the line 7 7 of FIGURE 4;

FIGURE 8 is a greatly enlarged cross-sectional view of any one rife roughly along the line 8 8 of FIGURE 4, it being understood that the section is normal to the riie even though line 8 8 traverses the ritile at an angle;

FIGURE 8a is a greatly enlarged cross-sectional view, similar to that of FIGURE 8, along the line de taa of FIGURE 4;

FIGURE 8b is a greatly enlarged cross-sectional View, similar to that of FIGURE 8, along the line db b of FIGURE 4; and

FIGURE 8c is a greatly enlarged cross-sectional View, similar to that of FIGURE 8, along the line sc c of FiGURE 4.

Although this invention may find application for the separation of any dry materials having a dierence in their specitic gravities sufficient for the concentration of one or more of such materials, such as seeds, coal and metallic ores, the operation of this invention will be described in relation to the separation of metals, by way of example, and it is to be understood that the invention is not limited merely to the separation of metals.

Since nature usually creates metals in an impure state, it is necessary to separate the impurities from the ores before the valuable metals can be smelted. Depending upon the nature of the ore, the mineral in which the metal is contained, the other minerals that are associated with it, and the grain size and texture of the mineral aggregate, a wide variety of treatment processes may become applicable in order to determine what can be done with the material after it has been mined. The science of metallurgy has developed a specialized knowledge for the determination as to what method of treatment is best suited to the ore, what portion of the metal can be recovered, and how much its recovery will cost.

Recovering a metal from its ore involves two kinds of proces-ses, one being purely physical, the separation of the metallic mineral from its gangue, and the other being chemical, breaking down the metallic mineral itself to recover the metal and get rid or" the other elements that are combined with it. The metal that either of these processes yield is rarely completely free from undesirable impurities; therefore, it needs relining before it is ready for the market.

Physical concentr tion, reduction, Iand refining are therefore, the three standard steps in the treatment of a typical ore; but one or even two of these steps can be omitted in special cases. If the ore is to be reduced by smelting, it usually pays to concentrate it at the mine before shipping it to the smelter, unless the `smelter is close at hand. It follows that the primary reasons for concentration are to avoid paying freight on valueless rock and to reduce the tonnage of material that is to be treated by the smelter. An additional reason is that metals in certain complex ores can be separated from each other more economically by milling than by smelting.

The ores that are fed to a mill are known as millheads or simply heads. The product from the mill is com- Leed Con tained, tons Percent Lead Gres Tous Millheads 100 The percentage recovery is 3.6 divided by 4.0, or 90 percent. The 0.4 ton of lead unaccounted for amounts to aY percent loss, presumably in the tailings, which should have a weight of 94 tons, excluding water, and thereforeassay 0.42 percent lead.

In a well-managed mill the heads and concentrates are systematically weighed and sampled as a matter of routine. The ytailings are sampled also; but, as they will have: been diluted by a large volume of water, except where a dry concentrating process is used, their weight is difliculttoV determinedirectly; therefore, their weight is usually estimated by deducting'the weight of concentrate from the weight of millheads. The results of these sarnples should check; that is, the weight of concentrates times the assay of concentrates plus the weight of tailings times the assay of tailings ought to equal the weight of millheads times theassay of mllheads. If this equation does .not balance, there is something wrong with the sampling orweighing, or with both. In small mines the discrepancy is likely to be found in the figures for millheads; where there is no automatic sampler, the millheads areY often sampled casually; and where there is no mechanical weighing device, their weight is commonly esti-y mated from the assumed Weight of a carload or skipload of ore, a factor that may lead to cumulative error unless it is checked periodically.

One method of ore concentration is hand picking. Primitive as it is, hand sorting can be the most economical method of concentration when circumstances favor it. Inits more modern form, hand picking is facilitated by'mech-anical aids. The ore, after coarse crushing, goes over a screen to separate-the fines Yand under a spray to wash off dust and mud. Then a broad conveyor belt, or, less commonly, a revolving table, carries it in front of the pickers. Used alone, hand picking is likely to be wasteful; if the valuable material is picked out of the waste, there will be execessive loss inthe residue. But, as a preliminary to mechanical concentration, hand sorting is often the cheapest method of separating ore from waste at coarse sizes. It may be employed either to get rid of part of 'the gangue and Wall rock or to collect pieces ofhigh grade ore for direct shipment. In either case, it reduces the bulk of the ore that has to be milled and so, in effect, increases the capacity of the existing treatment plant.

Another method of concentration is by the use of gravity. Gravity methods are mechanical refinements of the simpleV processes of washing and panning. Their effectiveness depends on the difference in specific gravity between different minerals; naturally, the greater the difference' the better the separation. Since a liquid buoys up a-body'by the weight of the liquiddisplaced, a particle immersed in' water has itsV apparent specific gravity reduced by l. That is, if asspecific gravity of a gangue mineral is 3 and that of a metallic mineral is 5, their relative Weights in water are asV 2 to 4.

But the size `as well as the specific gravity of a particle affects its behavior in a liquid. Large particles of light minerals settleas fast-as sm-all particles` of'heavy min# erals. For this reason separation is imperfect unless the particles all have the same size. Uniform sizing becomes especially important among fine particles. Among particles of very small size, gravity separation is not efiicient. Therefore, brittle minerals -that tend, on grinding, to yield a high prooprtion of slime give poor separation by gravity methods.

A great variety of machines has been used in gravity concentration, but much the commonest are jigs and vibrating tables. Auxiliary to these are boxes and cones of various forms designed to permit settling in an ascending current of water. Various types of gravity concentrators are used in combination withA other treatment' processes. Thus, ore in a relatively coarse state may pass thrdough jigs before oatation or cyanidation. The jigs take out large grains of heavy mineral and save the added expense and the possible tailing loss that would be incurred if this fraction of the material went through the complete process. Tailings from oatation or cyanidation are sometimes passed over slime tables toYpick-.up valuable particles that have escaped recovery. t

Another method of concentra-tion is by heavy huid sep aration. This is commonly referred to as the sinkandfioat process. This process operates most successfully'oii coarse ore from 2 inches down to 1A inch, but it has been used on'some types of ore as fine as 4'8mesh`wliich is about the coarser limit for with jigging, hand sorting, and the coarser ranges'rof tabling. The sink-and-fioat method isV best adapted to ore that breaks in such a wayA that Vthe valuable mineral or the gangue, or both, occur in chunks of fairly"la'rg size. For most ores it is a preliminary tofurthercon# centration by oa-tation or other methods. When" used as a pre-concentration process, it may serve either tore: cover a coarse marketable product, leaving a tailingthat can be further concentrated, or to reject coarsews'te and recover a low grade concentrate foradditional treatment.

Another method of ore concentration is the'oatation process. A particle of sulfide, suitably treated, ots on the surface of ywater whilefa par-ticle of quartz sinks.` This is because the quartz, unlike the sulfide, is wetted by the water; the sulfide adheres `to air andthe quartzto water. This same preferential adherence appliesv ntonly to mineral particles a-t the top of the liquid butv also yto the particles that are submerged. Thus, slfied particles adhere to bubbles of air and are buoyed upward'asthev bubbles rise to the surface. Although' each mineral be(- haves in its own' way with regard to adherence to air or water, the natural tendencies may be modified almost at will by introducingsuitable chemicals into the pulp (mixture of water and finely ground ore). The'practice is to mix the appropriate reagents into the pulp and cause air to bubble up through the mix-ture. rise with the air bubbles to form a froth whichoverows the tank and is filteredrto recover the mineral-bearing concentrate. l

Another method of concentration tion process. y gold and silver. This principle is utilized in the recovery of precious metals by passing a layer of pulp over a table consisting of a plate of silvered copper which hasbee'n coated with mercury. The mercury holds and partly'ab sorbs the particles of precious metals, While gangue and sulfides pass onward. The mercury and gold are later separated from each other by distillation of the amalgam. Amalgamation is a cheap and simple process and yields a product in-theform of directly marketable bullion. But it is suited onlyvto ores in which the gold or silver occurs in native and fairly coarse particles, asis the case in placers and in many oxidized deposits. Gold that vis locked up in grains of pyrite or other minerals escapes recovery. Hence, in modern lode mining, amalgamation is chiey an auxiliary process used to recover coarseV gold before cyaniding or floatation orto recver'free 'g'ld is by the amalgamafloatation. Thus, it competes The sulfide particles Mercury forms an amalgam with metallicV from floatation concentrates. It no longer has wide application in treating silver ores, since the proportion of silver occurring in native form is small.

Another method of ore concentration is by the magnetic separation process. Magnetic methods have long been used for concentrating magnetite or iron ores. The other iron oxides, hematite and goethite, as Well as carbonate and ciderite, are virtually non-magnetic, but they may be converted into artificial magnetite by controlled roasting. Magnetic separation occupies a limited but useful held chietly in the concentration or cleaning of ferrous ores and for separation of other magnetic materials that do not respond to floatation.

Mining, as most other businesses, is a matter of watching pennies. In the case of nearly all mines, two of the largest expenses are transportation and smelter costs. However, if the values in the native ore can be boiled down to a small part of its original bulk, the hauling and smeltering of useless mass can be eliminated, and these costs can be lowered to a point where they do not eat up most of the profits.

rhere is little problem where water is available; many different and varied water separators have been built to runswith a fairly high degree of retention and selection. Unfortunately, where there is no water, a mining property, unless it has exceptionally high values, cannot profitably operate, since there is no dry machine available to do what water does in more fortunately located mines.

Throughout the world are vast areas where water is not available but which contain large and rich ore deposits. These certainly would have been developed were Water near. Specically, in our country large parts of the West and the Southwest fall in this category. Known properties which from a cost standpoint today would be considered losing or marginal operations could be transformed into protable ventures with the advent of any efiicient dry ore concentrator.

In the last century, in this country alone, from S00 to G() different machines have been commercially developed to solve the problem of dry concentration. Most of these were unsuccessful, but a few have had a fair degree of etiiciency in percentage recovery, though not enough where volume was a factor.

By way of example, one of the earliest of the dry concentrators was a machine that depended upon gravity and an air current to separate the lighter material from the valuable metal desired to be recovered. The material was sized and sent to separators adjusted to the size; the sized material was fed onto a frame covered with a coarse heavy cloth, across which were rifles about four inches apart. The frame formed the upper side of a bellows that was turned by a crank having a Hy wheel. The puffs of air through the cloth agitated the gravel; and, aided by the slope of the frame, the waste was discharged at the lower end, while the heavier valuable material was retained in the riffles. The gravel and valuable material retained by the rifiies were brushed off into a tub, and, after a sutiicient quantity of this concentrate had accumulated, it was put over the machine a second time. The concentrate from the second operation was then separated into its various components, such as black iron sands from gold, by means of a magnet.

The art has demonstrated many improvements over this early type of dry concentrating table. Nevertheless, the dry machines that have put to use such improvements as have been developed heretofore have notably failed to achieve a percentage recovery of much more than 75 to 85 percent. In fact, even wet tables have rarely recovered more than 85 percent of the valuable ores present in the mixed materials being concentrated, except by the exercise of extreme and careful control, where 90 percent has been recovered by Water.

Routine runs on the dry table of the present invention have produced percentage recovery between 93 and 98 percent of the valuable ores.

It must be taken into consideration, however, that the dry table predominantly is not out to compete with the wet table, for the dry table is intended for use in areas where wet tables are geographically precluded. As a general rule, ores which are incompatible to a wet table are also incompatible to a dry table. However, the dry table does not slime as much as a Wet one; it does not lose the values of water soluble ores; and ores which tend to float due to their characteristics of water surface tension are unaffected on the dry table.

A preferred embodiment of the improved machine of the present invention comprises a standard-size separation table, four feet wide and eight feet long, whose bed is constructed of a porous material, such as cloth, porous tile, or tine wire screen, which allows air forced from underneath to permeate evenly Where its distribution is desired. This distribution is controlled by air Valves underneath the table bed. Along the top of the table are riiiies to hold the heavy materials. The table is vibrated longitudinally, and the vibrations are controlled as to speed and length of stroke. The whole table is angled laterally so that one side is higher than the other. Mixed material is fed at the upper side and head end of the table `by a hopper. When this material hits the table deck, the air forced through the deck has a tendency to make the material buoyant, and the heavier particles are retained by the riies and carried down to the foot of the table, while the lighter particles (gangue) cascade over the rifes and are discharged off of the lower side of the table. The overall eect is that the gangue is constantly eliminated over the lower side, and the concentrates ride the length of the table and are captured.

In order to agitate the table longitudinally, suitable means must be provided, such as an over-'balanced pulley working against springs yand a bumper, and affording a complete stop every time it makes one revolution. Such agita-ting means is common in this art and may be operated `by any standard motor suitable to the geographic location of the operation.

In order to provide uniform air pressure through the perforated deck of the table, it is necessary to produce a :back-pressure in the air chamber. Also, depending on the type of ores and the fineness of grind, there must be a suitable means for regulating the uniform air pressure so as .to provide more air pressure when the millheads constitute a coarser grind and less air pressure for the more fine grind of ores. It is to be understood that any of the suitable constructions common and available for performing these functions of producing and regulating uniform air pressure may be utilized.

The improvement of 4the present invention lies primarily in the novel construction of the riffles in com- Ibination with certain preferred features in themselves old in the art of dry concentrators. These riies may be constructed of any suitable material, such as metal, wood, or any of the plastic bers, and may be suitably aligned along the deck in parallel spaced relationship longitudinally or at any angle to the dimensions of the table. It is preferable that the ritiies be of graduated lengths, those at the upper side being shorter at the discharge end than Ithose at the lower side, so as to permit any gangue retained vby the rflie to flow laterally; however, it is to be understood that this aspect is only preferential and not a limitation. It is also preferable that gates or dams running normal -to the aligned rimes be eliminated so as to avoid any damming up and re-mixing of the material after it has begun to concentrate.

Many of the riies of the prior artt are too high and sharp. Most ores are granular and have sharp edges like facets in crystals. In order to more efficiently hold the concentrate and let the lighter gangue flow over, the present invention embodies a riifle having a longitudinally -r these ores to the deck of the separating table.

Vritlies, and any other construction of partially-tapered riHes as well.

Thus, the present invention entails the construction -of a dry concentrator for the separation of valuable material and gangue from millheads, comprising, in combination with a laterally-inclined separating table, a plurality `of aligned, spaced rifes mounted on Said table, each rife being tapered toward its foot and having an upper leading edge that is bevelled so as to retard the action of air'oatation in relation to such valuable' ma- -terial as would otherwise be dissipated and lost inthe tailings.V

Just as some gangue may be retained by the riihes Vand -caused to transverse the table longitudinally, some of the concentrates are blown over the riilles as a result of the air oatation and are passed oit into the tailings and lost. The percentage loss of valuable material has been found to be -as high as from l() to percent with a ritlie construction as in the prior art, but has been reduced to -form 2 to 7 percent by machines utilizing the teaching of the present invention.

One explanation for this result may well be that such concentrate as is blown up by the air iioatation process passes over the bevelled'riflie, thereby is removed from the `air stream and allowed to fall on the riiiie, Where a portion will be retained by the bevelled edge and passed longitudinally, being substantially unaffected by the air lloatationprocess while so retained. It will lthus be seen vrthat some concentrate will be retained by each riflle base, some concentrate will be retained by the bevelled edge on the riftle, some concentrate will pass over the rife for similar action in Vrelation to the succeeding rifiles, and ultimately some concentrate will be lost inthe tailings. c Y

Another explanation'for this result, which is not entirely understood by the inventors, is that static` electricity may well be set up as a result of the airY passing through the deck under pressure and up through theore carried thereon. This static electricity has a tendency to attract most ores that have a small degree of magnetic qualities in them, such as tungsten, Vmagnetite iron, manganese, garnet, gold with iron, and'others, and to hold As illustrated in FIGURE 1 of the drawings, it has been found that the concentrates that have passed in a longitudinal t direction beyond the bounds of the upper, shor-ter ries Ystay Yat substantially the same `lateral position high on the deck while continuing to move longitudinally, desipte the absenceY ofY rililes.

TheY direction of movement of the Vconcentrates is illustrated by the line 9b-9b,in FIG- URE 4. It is believed that the static friction builtnp, as described, acts to hold the concentrates in relatively the same position onY the table laterally, whilethe reciprocal motion of the table vkeeps the concentrates moving along the table longitudinally.

'Referring now more particularly to the drawings, FIG- URES 1-3 show one preferred embodiment of the present invention, in which the dry concentrator is denoted Vgenerally by the reference number 10. YThe dry concentrator 10 comprises a separating table 11 Vhaving a deck -12 along which are aligned arplur-ality of spaced-apart Yriiies 1'5. VThe deck 12 is supported by side members 14 and e ad members 15 suitably fastened together to form the tzlblejll in a structurally sturdy manner. The table 11 v*is suitably supported on a frame 16 mounted upontlexi- `ble legs 17 which are in turn mounted on a rplatform 1%. The table 11 is articulated with relation to the frame 16 and may be elevated on one side by means of a crank 19 controlling elevating mechanism (only partially shown) of any suitable design. In the illustrated embodiment the elevation of the table 11 is controlled by a guide member 2i) mounted on the platform 18 and having suitable guide slots Z1, in combination with depending carriage mem- :bers 22 mounted on the table l11 and cooperating with the guide slots 2.1. The carriage members 22 ride upon the guide members 2G; and the extent of upper elevation is determined by the height of the guide member, while the extent of lower elevation is limited by a bumper 23 in each slot 21.

The tiexible legs 17 f provide leeway for longitudinal movement of the tabie relative to the platform 1S. Any suitable means known in the art for agitating or Vibrating the table, such as an electric or gasoline motor 24 and an over-balanced pulley (not shown) working against springs and a bumper (not shown), may be provided to reciprocate the table longitudinally. The table is brought abruptly to a complete stop every time the agitating means ymakes one revolution; it is operated at between 20() and 500, and preferably 300 cycles per minute.

Any suitable means may be provided for maintaining uniform air pressure beneath the deck 12. in the preferred embodiment shown in the drawingsfa motor driven compressor 25 draws air from the ambient atmosphere and impels it through :a central tubular member26 from which the air is circulated through supplementary hoses 27 and introduced through gaskets 28, connected with openings 28a in a floor 29 of the table 11, into an air chamber 30 below the deck 12. The construction of the table 11 is such as to pennit the -air pressure within the air chamber 30 to build up to a desired value so that, when the air ows through the table deck 12, it is released at a uniform total pressure over the sur-face of the deck 12. However, since diiferent types of ores require different degrees of air pressure depending upon the nature and size of the ore, any suitable regulating means may be provided in combination with the compressor 25 to'produce the desired degree of pressure within the air cham:- ber 30.

The table deck 12 is air-permeable and may be of any suitable construction, although it has been found that a layer of glass fibers or other building insulation material, having a thickness of approximately l inch, upon which has been stretched a cloth `of metal or fiber, having a mesh of minus 260, will operate in Ia manner to provide suitable back-pressure Vof the air within the chamber-30. The deck 12 may be supported by ribs 31 of a thickness comparable with that of the ritlles 13 and which are mounted in alignment with and'directly beneath each rfic. These ribs 31 are in turn supported by cross rails 32 within the table 11. Such a construction provides an open chamber 3G and avoids the appearance of blank spots, other than at the riies, on the table deck 12, which blank spots would otherwise produce undesirable whirlpools and tornados in the material being separated.

The rimes-'13 may be constructed of any suitable material, such as metal, Wood or liber, and are aligned alongV the deck 12 of the table 11 in spaced, parallel relation,

running from the head 33 toward the foot 34. ThoseA rifes nearer to the upper side 35 of the table 11 are :shorter than those nearer to the lower side 36. This is to permit such gangue as is retained by the upper riiiles to oW laterally off the lower side 36 of the table V11.

Each rile 13 may be tapered toward the foot of the Ytabley to further aid in the release of gangue. In the `preferred embodiment shown in the drawings, the riiiles begin tapering at a point approximately 2 feetfrom the end of'each j g termediate points along the lines Sa--Sa and S17-8b of FIGURE 4, and are illustrated in the corresponding figures. At the same time, the leading edge 37 of each riie 13 is bevelled, extending throughout the length of the ritlie. This bevelled surface 38 is shown more clearly in FIGURES 7-8c. A trailing or lower edge 39 of each riflie is normal to the base 40 of the rie 13. In the operation of the dry concentrator 10, the table deck '12 is adjusted to the desired angle of inclination depending on the type of materials being concentrated. It will thus be seen that the bevelled surface 38 of the ries 13 is substantially in a plane parallel to the ground although it may vary -from this horizontal position depending upon the adjustment of the deck 12 of the separating table 11.

Suitable means for catching the separated materials may be provided, such as a tray 41 at the lower side 36 of the table to catch the tailings, a tray 42 at the lower foot 34, 36 of the table to catch the middlings, and a tray 43 at the upper foot 34, 35 of the table to catch the concentrates. The mixed ores are introduced at the upper head end 33, 35 of the table by means of a suitable hopper 44; and -a suitable railing 45 may be provided at the head end 33 and extending around the upper and lower sides 35, 36 to prevent any discharge Where there is no means provided for catching the same. An apron 46 may be provided at the foot 34 of the table to assist in discharging the concentrates and middlings.

Another embodiment of the present invention proposes a saving in energy input by the provision of suitable means to block ott those portions of the deck 12 where the materials to be separated do not normally tlow. As will be seen from the drawings, the material usually iiows from the upper head end 33, 35 of the table 11 toward the lower foot end 34, 36 of the table, and little if any material comes in contact with the lower head end 33, 36 or the extreme upper foot end of the table 11. A substantial saving in energy will 'be entailed by either blocking the air-permeable deck in these pertinent areas or by providing a non air-permeable deck at such areas. It will also be seen that the riii'les may be shortened at the lower head end 33, 36 just as they are shortened at the upper foot end 34, 35 as illustrated in the drawings. This modiiication may be made for the same reasons that the deck is non air-permeable in this area, that is to say, that the simplicity of construction will work a saving in the ultimate cost of operation of this machine.

Although the operation of this invention has been described in relation to the recovery of heavier valuable concentrates and the disposal of lighter weight materials, it is to be understood that some valuable ores have a lighter specific gravity than the gangue with which it is disposed. An as example, in order to recover mica, which has a relatively light Weight, from its ore, such recovery would be instituted by capturing the lightest portion of the tailings coming off the lower or tail side 36 0f the table. The tailings discharged from the middle of the lower side 36 along the line 9a-9a have a tendency to be lighter than the tailings coming ot of the lower corner 34, 36 of the table. By making a division of the tailings such light materials as mica may be readily recovered from the ores.

Another embodiment of the present invention entails the addition of a cut-o. rifle (not shown) which might be mounted on the deck 12 along the line 9-9 of FIG- URE 4. Such Va cut-oit Iiie would be of a similar construction to the other rifdes of the machine and would additionally aid in the division of concentrates and middlings. p

As examples of the percentage recovery produced in machines utilizing the teaching of the present invention, the following results from actual test runs and laboratory analyses are given:

Example 1 400.5 pounds of gold placer sand having a size of minus 20 plus 60 mesh was introduced to the dry concentrator as millheads. Operation of the machine produced 10.75 pounds of concentrates or 2.7 percent of the original Weight of the millheads; 74 pounds of middlings or 18.5 percent of the original weight; 305 pounds of tailings or 76.1 percent of the original weight; and 10.75 pounds or 2.7 percent by weight were lost. The test run lasted `for l2 minutes at the rate of l ton of ore per hour. Assaying determined that percent of the `gold in the millheads was 0.01 ounce per ton of ore; the concentrates assayed at 0.33 ounce of -gold per ton of ore or 88.5 percent of the total gold values represented; the middlings assayed at 0.0025 ounce per ton or 4.6 percent of the gold values represented; the tailings assayed at nil; and a gold loss of 6.9 percent was incurred. Since the middlings are passed through the concentrator a second time, the total percentage recovery of gold for this test run was 93.1 percent, being the sum of the recovery percentages for the concentrates and middlings.

Example 2 This test run was made on 292.75 pounds yof tungsten ore, containing both scheelite and hubnerite minerals, ground to minus 20 plus 60 mesh. After seven minutes of operation at the rate of 1.25 tons of ore per hour, the concentrates were determined to Weight 17.75 pounds or 6.1 percent of the original weight of the headings; the middlings weighed 44.25 pounds or 15.1 percent of the original weight; the tailings weighed 224.25 pounds or 76.6 percent of the original Weight; and there was a loss of 6.5 pounds or 2.2 percent of the original weight. 'I'he heads assayed 3.85 percent tungsten (W03), being 100 percent of the values represented; the concentrates assayed 57.95 percent tungsten or 91.3 percent of the values represented; the middlings assayed 1.35 percent tungsten or 5.3 percent of the values represented; the tailings assayed 0.15 percent tungsten or 3.0 percent of the values represented; and the loss amounted to 0.4 percent of the values represented. Thus, the total percentage recovery (concentrates plus middlings) was 96.6 percent.

It will be apparent from the foregoing description and explanation that the invention provides a dry concentrator having numerous advantages over dry concentrators yof this type previously available.

While the arrangement of the invention described herein is at present considered to be preferred, it is understood that variations and modifications may be made therein, and it is intended to cover in the appended claims all such variations and modifications as lfall within the tnue spirit and scope of the invention.

What is claimed is:

l. In a dry concentrator, a generally horizontally disposed separating table adapted to be longitudinally reciprocated, said table having a head end and .a foot end, means for inclining said table to the horizontal along a longitudinal axis so as to establish a lower side and an upper side thereof, an air permeable deck on said table, means to supply air under pressure beneath said deck, a plurality of riies mounted on said deck and extending longitudinally of said table, said ricles being laterally spaced a predetermined distance apart on said deck, each of said rittles having an upper surface bevelled such that said surfaces lie in generally horizontal planes when said table is inclined.

2. In a dry concentrator, a generally horizontally disposed separating table adapted to be longitudinally reciprocated, said table having a head end and a foot end, means for inclining said table to the horizontal along .a longitudinal axis so as to establish a lower side and an upper side thereof, an air permeable deck on said table, means to supply air under pressure beneath said deck, a plurality of riles mounted on said deck and extending longitudinally of said table, said Iiies being laterally spaced a predetermined distance apart on said Vbevelled such that said surfaces lie in generallythorizont-al planes when Ysaid table is inclined. f i Y 3. The dry concentrator of claim 2 further characterized in that each tapering section comprises =an end section of .a corresponding vriiie adjacent said foot end.

4. In a dry concentrator, a generally horizontally disposed separating table having a head end and afoot end and adapted to be longitudinally reciprocated, means for inclining said table to the horizontal about a longitudinal axis so as to establish a lower side and yan upper side thereof, an air permeable deck on said ytab1e, rneans to supply air under pressure beneath said deck, a plurality of riies mounted on said deck and extending longitudinally of said table in spaced relationship -a predetermined .distance from each other, the ends of said riles 12 adjacent said foot end terminated progressively further from said foot end between said lower side and said upper side, a section of each of said riles tapering toward said i-foot end, said tapered sections being of gener-ally equal length, each of said ritles having an upper surface bevelled such that said surfaces lie in `generally horizontal planes when said table is inclined.

References Cited in the tile of this patent UNITED STATES PATENTS 988,748 Wall -L Apr. 4, f1911 1,603,997 Sutton et al Oct. 19, 1926 1,812,071 Arms June 30, 1931 Y FOREIGN PATENTS 170,174 Austria Jan. 25, 1952 

1. IN A DRY CONCENTRATOR, A GENERALLY HORIZONTALLY DISPOSED SEPARATING TABLE ADAPTED TO BE LONGITUDINALLY RECIPROCATED, SAID TABLE HAVING A HEAD END AND A FOOT END, MEANS FOR INCLINING SAID TABLE TO THE HORIZONTAL ALONG A LONGITUDINAL AXIS SO AS TO ESTABLISH A LOWER SIDE AND AN UPPER SIDE THEREOF, AN AIR PERMEABLE DECK ON SAID TABLE, MEANS TO SUPPLY AIR UNDER PRESSURE BENEATH SAID DECK, A PLURALITY OF RIFFLES MOUNTED ON SAID DECK AND EXTENDING LONGITUDINALLY OF SAID TABLE, SAID RIFFLES BEING LATERALLY SPACED A PREDETERMINED DISTANCE APART ON SAID DECK, EACH OF SAID RIFFLES HAVING AN UPPER SURFACE BEVELLED SUCH THAT SAID SURFACES LIE IN GENERALLY HORIZONTAL PLANES WHEN SAID TABLE IS INCLINED. 